Due to widely spreading new services such as distribution of video pictures making use of cloud computing on the Internet, communication traffic is expected to rapidly increase. In order to deal with the continuously increasing communication traffic, research and studies are made on optical transmitters and receivers capable of signal transmission at a rate of 100 Gbps or more.
However, if the bit rate per wavelength is increased, the signal quality is degraded due to degradation of the optical signal to noise ratio (OSNR) performance or waveform distortion caused by wavelength dispersion, polarization mode dispersion or nonlinear effects in the transmission path. Addressing this issue, in recent years and continuing, digital coherent receiving technology is attracting much attention because of superiority in OSNR performance and resistance to waveform distortion. (See, for example, Non-patent Document 1 listed below).
With a digital coherent receiving technique, OSNR performance is improved, and compensation for waveform distortion and adaptive equalization with respect to time-varying propagation characteristic of optical transmission paths can be realized using a digital signal processor. Accordingly, high performance can be maintained even in high-bit-rate transmission. Unlike conventional intensity-modulated direct detection allocating on/off states of light intensity to a binary signal, a coherent receiving technique extracts intensity and phase information and quantizes the extracted intensity and phase information at an analog-to-digital (A/D) converter. The quantized information is demodulated at a digital signal processor.
When DP-QPSK (dual polarization—quadrature phase shift keying) is employed as a phase modulation scheme, two-bit data states are allocated to four optical phases (0°, 90°, 180°, and 270° for each of two orthogonal polarized waves (polarized along the x axis and the y axis). The symbol rate can be reduced to ¼, and accordingly, the system can be made smaller and the cost can be reduced.
A light signal having been propagated through an optical fiber is separated into horizontal polarization component (H-axis polarization) and vertical polarization component (V-axis polarization) before the light signal is input to a digital signal processor. Each of the H-axis and V-axis polarization components is detected by a local oscillating laser with 90-degree phase shift, separated into an in-phase channel and a quadrature channel, and subjected to analog-to-digital (A/D) conversion. Because transmission-side polarization along the X axis and the Y axis is not in accord with receiving-side polarization along the horizontal axis (H axis) and the vertical axis (V axis), and because polarization mode dispersion exists in optical fibers, X and Y components of the transmitted signal are generally mixed into the H and V components of the received signal. The X component and the Y component of the transmission signal are separated from the H component and the V component of the received signal by an adaptive equalized of a digital signal processor. The adaptive equalizer also equalizes waveform distortion caused by band limitation due to wavelength division multiplexing, polarization mode dispersion or residual wavelength dispersion (which is a residual component of waveform distortion compensation).
Since propagation characteristics of an optical fiber change due to vibration or temperature change, adaptive equalization is demanded not only in the initial training period, but also during communications (data transmission). Accordingly, tap coefficients are calculated and updated taking as many input signals and output signals as required into calculation so as to satisfy the necessary follow-up rate (the maximum of characteristic changing rate of transmission path).
In order to prevent the X-branch and the Y-branch from converging to the same information source, it is proposed to calculate filter coefficients by generating a new set of filter coefficients for one of the X and Y branches based upon the output of filter coefficients of the other branch. (See, for example, Patent Document 1). With this method, a symmetry center of the filter coefficients of one of the branches is calculated, and the filter coefficients are folded back at the symmetry center. Then complex conjugate permutation is performed on the filter coefficients having been subjected to the foldback process to acquire a new set of filter coefficient for the other branch. When calculating the symmetry center, centers of electric power of the Hxx filter and the Hyx filter are calculated respectively, and the average of the centers of electric power is selected as the symmetry center.
Patent Document 1:    Japanese Laid-Open Patent Publication No. 2009-253972
Non-Patent Document 1:    D. Ly-Gagnon, IEEE JLT, vol. 24, pp. 12-21, 2006